The proposed project provides a means for studying the regulation of renal calcium transport. The study will employ the technique of in vivo microperfusion of the distal tubule, where calcium transport rates can be measured directly, at constant delivered load. Experiments will examine the hormonal regulation (parathyroid hormone, anti-diuretic hormone, aldosterone) of distal calcium transport. Particular attention will be given to the sites and mechanisms of action, and to the effects of the hormones on the relationship of calcium to sodium transport. Other experiments will look at the mechanism of calcium-retention by distal diuretics. Additional experiments are designed to examine the mechanisms of the hypercalciuria of phosphate depletion and metabolic acidosis. It is anticipated that the data generated will provide new information on the sites and mechanisms of hormonal modification of renal calcium reabsorption, a project with practical implications for understanding complications of renal disease and disorders of calcium metabolism. In addition experiments dealing with the modification of calcium transport by phosphate depletion, acidosis and certain diuretics should provide insight into the pathophysiology of renal stone formation and its therapy.